One known method for abating certain diesel engine exhaust constituents is by use of an exhaust after-treatment system that utilizes Selective Catalytic Reduction (SCR) of nitrogen oxides. In a typical SCR system, urea or a urea-based water solution is mixed with exhaust gas. In some applications, a urea solution is injected directly into an exhaust passage through a specialized injector device. The injected urea solution, which is sometimes referred to as diesel exhaust fluid (DEF), mixes with exhaust gas and breaks down to provide ammonia (NH3) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NOx) in the exhaust at a catalyst to provide nitrogen gas (N2) and water (H2O).
In typical applications, especially for large engines, high efficiency diesel particulate filters (DPF) are used in conjunction with NOx reduction systems such as systems using SCR. Such systems are generally quite effective in filtering soot while also converting NOx emissions from diesel exhaust, but such systems are also relatively large in volume. For example, a typical combined DPF/SCR after-treatment system, which may also include AMOX and DOC catalysts, can be approximately 3-6 times engine displacement in volume, which makes it challenging to design and integrate into a vehicle or engine system and also increases overall machine weight and cost.
It has been proposed in the past to coat the SCR catalyst onto the DPF filter substrate to eliminate a separate substrate for the SCR catalyst and allow DEF injection upstream of the DPF, but the low temperature soot oxidation reaction and fast SCR reaction will compete for NO2 during engine operation, which will generally result in high DPF balance points, i.e., a system balance at high soot loadings on the DPF, which is known to make the DPF prone to cracking or catastrophic failure, and requires DPF regeneration at a high temperature. High temperature regeneration often requires so-called active regeneration, which entails conducting the regeneration using a heat source or a high fuel concentration, both of which reduce fuel economy for the machine.
One example of a previously proposed after-treatment system can be seen in U.S. Pat. No. 8,413,432 to Mullins et al. (“Mullins”). Mullins describes a regeneration control system for a vehicle that includes a regeneration control module and a regeneration interrupt module. The regeneration control module selectively provides fuel to an oxidation catalyst for a regeneration event of a particulate filter that occurs during a predetermined melting period for frozen dosing agent. The regeneration interrupt module selectively interrupts the regeneration event and disables the provision of fuel to the oxidation catalyst before the regeneration event is complete when a temperature of a dosing agent injector that is located between the oxidation catalyst and the particulate filter is greater than a predetermined temperature. As can be appreciated, therefore, the system of Mullins requires active regeneration.